Romney’s strength is that he makes a great first impression—he seems reasonable, confident, and competent. Attacking him as a “flip-flopper” only reinforces that view, and makes him seem less threatening to voters who aren’t sure that they want to support a Republican. Alternatively, by attacking him as too conservative, Democrats remind the public of its dislike for the GOP, and tie Romney to the most unpopular elements of the conservative agenda. It also helps that the former Massachusetts governor is also working to define himself as a conservative, in order to secure support from the Republican base.